With the use of personal smart devices as personal communication and entertainment devices becoming ubiquitous, audio privacy is increasingly also becoming a serious concern. In most instances where audio sound is openly played through loudspeakers of smart devices (e.g. via a “speakerphone”, where a smartphone is typically placed at a distance of about 20 cm away from a user's mouth), a drawback is that there is little audio privacy, and also often creates “noise pollution” to bystanders. Put simply, the audio sound played through the smart devices may be heard desirably by an intended listener, and also undesirably by the bystanders, since they may have no interest to listen in.
One way to address the above issue is to use parametric loudspeakers. Compared to conventional loudspeakers (which are typically installed in modern smart devices), parametric loudspeakers feature significantly improved audio privacy, since audio sound generated by the parametric loudspeakers is highly directional and so, a private audio zone may accordingly be created. However, at this juncture, parametric loudspeakers are typically employed only in outdoor applications, but not for portable smart devices. Reasons why that is so include due to high power dissipation, subdued low frequency response, and poor directivity of parametric loudspeakers (if a size of the associated transducer is reduced, e.g. to less than 10 cm2). On the high power dissipation issue, it has been determined that more than 5 W of power is needed to operate parametric loudspeakers to generate about 63 dB Sound Pressure Level (SPL) at a distance of 50 cm at 1 kHz, whereas to generate audio sound of comparable loudness using traditional audio loudspeakers, only a power of less than 1 W is required. On the directivity issue, in order to ensure a reasonable directivity, a size of the transducer for the parametric loudspeakers needs to typically be greater than 25 cm2.
The operation principle of parametric loudspeakers is described below. A parametric loudspeaker is configured to emit a beam of amplitude-modulated ultrasonic signal, which is then demodulated in the air, thereby generating a desired audible audio signal. As the frequency of the ultrasonic signal is high (i.e. typically greater than 40 kHz), the ultrasonic signal is thus highly directional and consequently the demodulated audio signal is also highly directional, resulting in creation of the private audio zone.
More specifically, FIG. 1 shows schematics of a parametric loudspeaker system 100, which includes a signal processor 102, a carrier generator 104, a (Class D) power amplifier 106, and a parametric emitter 108 (or otherwise known as a parametric loudspeaker). The signal processor 102 first processes an audio input signal 110 and generates a desired envelop signal of the audio input signal 110. The envelop signal modulates an ultrasonic frequency carrier signal provided by the carrier generator 104, and an amplitude modulated ultrasonic signal is then generated. Depending on the type of ultrasonic emitter used in, and a design of the parametric emitter 108, a carrier frequency of the generated ultrasonic signal may range from 30 kHz to 200 kHz. The power amplifier 106 serves to amplify the ultrasonic signal and provide sufficient current to drive the parametric emitter 108. By means of transduction, the parametric emitter 108 generates the acoustically modulated ultrasonic signal. The ultrasonic signal is demodulated in the air, and the desired audible audio signal is obtained. The parametric emitter 108 may be realized using a bimorph transducer (not shown) or an array 200 of ultrasonic transducers 202 (i.e. refer to FIGS. 2a and 2b). For the latter configuration, the ultrasonic transducers 202 are positioned in a honeycomb-like arrangement and connected in parallel. The purpose of having multiple ultrasonic transducers 202 is to increase a sound pressure level of the generated ultrasonic signal (i.e. which is outwardly emitted in a direction from a transmission plane of each ultrasonic transducer 202), hence increasing an acoustical loudness of the demodulated audio signal.
However, it is to be appreciated that conventional parametric loudspeakers nonetheless suffer from some drawbacks which include:
High Power Dissipation
Reasons for the high power dissipation include: (i) requiring generation of high ultrasonic Sound Pressure Level (SPL) for obtaining an audio signal with sufficient loudness, and (ii) suffering from low power-efficiency by the power amplifier 106. The former reason is due to the highly inefficient demodulation process in the air. For instance, to generate a 66 dB audio signal, a 130 dB ultrasonic signal is needed, meaning that the attenuation losses in reality are fairly high (i.e. 130 dB—66 dB=44 dB). When translated to power terms, this means that approximately 5 W of ultrasound power is needed to obtain an equivalent of 1 W of audio power. On the other hand, reason (ii) is mainly due to the high-switching frequency of the conventional power amplifier 106. It is highlighted that amongst the various components of the parametric loudspeaker system 100 depicted in FIG. 1, the power amplifier 106 is the most power dissipative component, and dissipates about 90% of the total power used by the system 100. So, to reduce the overall power dissipation of the system 100, it is important that the amplifiers used therein have high power-efficiencies. In this respect, Class D amplifiers are usually employed for their significantly higher power-efficiency compared to their linear counterparts. But, unlike Class D amplifiers used in traditional audio loudspeakers whose power-efficiency is usually very high (i.e. greater than 95%), the power-efficiency of conventional Class D amplifiers deployed for parametric loudspeakers tends to be relatively low, typically less than 80%. The low power-efficiency is due to a high supply voltage used (i.e. greater than 20 V) and high switching frequencies (i.e. greater than 400 kHz) of the Class D amplifiers, which may also generate undesirable electromagnetic interferences (EMI).
More specifically, conventional Class D amplifiers tend to be configured with a high switching frequency—typically about 10 times higher than the bandwidth of the amplifiers. The high switching frequency consequently results in high power dissipation in the Class D amplifiers. This is especially so for parametric-loudspeaker applications because of the wide bandwidth (i.e. which is greater than a carrier frequency of the parametric loudspeaker, for example greater than 40 kHz) and the high supply voltage used for the Class D amplifiers. A conventional Class D amplifier 300, coupled to a parametric emitter 301, is depicted in FIG. 3 for reference sake. The conventional Class D amplifier 300 comprises an integrator 302, a triangular wave generator 304, a pair of comparators 306a, 306b, and two Class D output stages 308a, 308b. The integrator 302 amplifies an input (Vin) and feedback signals, and provides a high loop gain in the feedback branches. The triangular wave generator 304 generates a triangular waveform, and the pair of comparators 306a, 306b then compares the triangular waveform with respective outputs (Vint1, Vint2) from the integrator 302 to generate corresponding Pulse-Width-Modulated (PWM) switching signals. A switching frequency of the switching signals (or a frequency of the triangular waveform) is typically 10 times higher than the input signal (Vin). In the case of parametric-loudspeaker applications, the switching frequency is typically about 400 kHz. As aforementioned, power amplifiers typically dissipate the most power, and the power loss incurred is largely proportional to a switching frequency of the power amplifiers. So, to improve the power-efficiency of the Class D amplifier 300, it is important that the switching frequency of the Class D amplifier 300 is sufficiently low. For completeness, it is to be appreciated that an inductor 310 (Ls) and a resistor 312 (Rs), coupled to respective outputs of the two Class D output stages 308a, 308b, are configured to filter any switching components of the Class D amplifier 300, but may optionally be omitted depending on a design of the Class D amplifier 300 and technical characteristics of the parametric emitter 301.
Limited Directivity
Limited directivity happens when a size of a parametric emitter is arranged to be dimensionally small. Particularly, the directivity of an audio beam generated by a parametric emitter largely depends on a transducing area, and a specific carrier frequency of said parametric emitter. A higher carrier frequency results in better directivity, but however incurs higher power dissipation by the parametric emitter. It has been shown that having a carrier frequency of about 40 kHz presents a rather good trade-off between the power dissipation and directivity issues. Also to ensure a good directivity, a size of the transducing area of a parametric emitter preferably should be greater than 25 cm.
One object of the present invention is therefore to address at least one of the problems of the prior art and/or to provide a choice that is useful in the art.